In many industrial applications involving heat transfer from flue gases to a clean gas flowing inside tubes, it is desirable to improve the tube-side heat transfer coefficient. For instance, in heat exchange tubes in high-temperature applications, such as fluidized bed combustors, with the combustion products on the outer wall, there is a large difference in the heat transfer coefficients between the inner and outer walls. Published literature, for instance an article entitled "Future Development of High Temperature Heat Exchangers" by Y. Morri, Department of Mechanical Engineering, University of Electro-Communications, Chofugaoka, Chofu, Tokyo, Japan 182, teaches a 10% to 70% increase in heat transfer coefficients by the use of twisted tape or cross tape positioned inside the tubes. In other conventional methods, the heat transfer coefficient is attempted to be increased by raising gas velocity, the use of rough internal surfaces for turbulent flow and the use of fins. These methods incur an off-setting penalty of increased pressure drop or material cost.
In the subject invention, an alternative method and structure is provided which produces increased heat transfer coefficients using radiative means. Normally, radiation is not an effective mode of tube-side heat transfer because the mean beam length is small, on the order of 2.5 to 7.6 cms, and because the gas may be transparent to thermal radiation. To create a participating medium, thin filaments are dispersed inside the tubes of the heat exchanger. The filaments are heated radiatively by the tube surface and, thereafter, transfer the heat convectively to the flowing gas inside the tubes. By this means, the filaments are nearly isothermal with the gas by virtue of the high heat transfer coefficient and the extended surface area obtained with the use of thin filaments.